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Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area, Central Eastern Desert, Egypt
The Egyptian Journal of Remote Sensing and Space Sciences (2012) 15, 171–184
National Authority for Remote Sensing and Space Sciences
The Egyptian Journal of Remote Sensing and Space Sciences www.elsevier.com/locate/ejrs www.sciencedirect.com
Research Paper
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area, Central Eastern Desert, Egypt A.K.A. Salem a, A.E. Khalil a, T.M. Ramadan a b
b,*
Department of Geological Sciences, National Research Centre, Egypt The National Authority for Remote Sensing and Space Sciences, Cairo, Egypt
Received 17 November 2011; accepted 30 November 2011 Available online 20 December 2012
KEYWORDS Tectonic setting; Pan-African; Serpentinites; Podiform chromite; Subduction zone; Ultramafic cumulate
Abstract This research aims at integrating remote sensing data and field studies for lithological mapping of different rock units. Geological interpretation of Landsat ETM+ image and field studies revealed that the study area is mainly covered by ophiolitic ultramafic rocks, ophiolitic metgabbros, schistose metavolocanics and alkaline granites. The serpentinites masses occur in the form of dipping or steeply inclined sheets or lenses generally emplaced along thrust faults .They are elongated in a ENE-WSW direction. They are composed essentially of antigorite, clinochrysotile and lizardite with subordinate amount of magnesite, chromite, magnetite, talc and chlorite. Geochemically, the studied serpentinites are rich in MgO, Cr, Ni and high Ni/Co ratio. The examined serpentinites belong to the ultramafic cumulate ophiolite rocks and it was originally a harzburgite that represents a fragment of the oceanic lithosphere formed in a back arc basin, i.e., they belong to an ophiolitic mantle sequence which was formed above subduction zone (SSZ). Several mineral deposits are associated with the ophiolitic ultramafic rocks such as magnesite, talc and chromite. Chromite lenses are mainly encountered at wadi level in a podiform shape. Microprobe analyses of chromite show that the chromite is an aluminum chromite type. The podiform chromite is characterized by low TiO2 (<0.3%) and MnO (<0.27% contents) which are considered as distinct feature of Alpine type chromite deposits. The presence of podiform chromites in
* Corresponding author. E-mail address: [email protected] (T.M. Ramadan). q Peer review under responsibility of National Authority for Remote Sensing and Space Sciences
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172
A.K.A. Salem et al. the serpentinites are related typically to supra subduction zone ophiolites (SSZ). The studied serpentinites show similarities to the Alpine type serpentinites. Ó 2011 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction Serpentinites are widely distributed in the ophiolite assemblage that constitutes an important part of the basement complex of the Eastern Desert of Egypt (Takla et al., 1975; Garson and Shalaby, 1976; El Sharkawy and El Bayoumy, 1979; Takla and Noweir, 1980; Amstutz et al.,1984; Habib et al.,1987; El Gaby et al., 1988, 1990; Hilmy et al., 1991; Khalil, 1994 ; Khalil et al., 1996; Akaad, 1997; Akaad and Abu Ella, 2002; El Bahariya and Arai, 2003; Salem et al., 2004; Azer and Stern, 2007; Khalil and Azer, 2007; Abd El-Rahman et al., 2009; Abu El Ela and Farahat, 2010). Um Salim-Um Salatit area is located in the Central Eastern Desert between Lat. 25°50 –25°320 N and Long. 33°520 –34°020 E (Fig. 1). The present paper deals with detailed geological, mineralogical and geochemical studies of the ultramafic rocks exposed in the study area.
Figure 1
These serpentinites form huge lenticular bodies with ENE– WSW trend, conformable with the regional strike of the host rocks. 2. Methodology 2.1. Remote sensing data In this study, one scene of Enhanced Landsat ETM+ data (Path 174 and Raw 42, acquired on March, 2000) covering the investigated area, has been geometrically corrected and radiometrically balanced using the ERDAS imagine 9.1 at the National Authority for Remote Sensing and Space Sciences (NARSS), Cairo. The applied techniques include false color composite image (bands 7, 4, 3, Fig. 2) to produce structural map and rationing image (bands 5/7, 5/1, 4, Fig. 3) to distinguish the different rock units.
Landsat ETM+ image showing the location of study area (white box).
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
Figure 2
Figure 3
False colour Landsat ETM+ image (bands 7, 4, 3 in R, G, B) for the study area.
Landsat ETM+ ratio image (bands 5/7, 5/1, 4 in R, G, B) for the study area.
173
174 Table 1
A.K.A. Salem et al. X-ray diffraction data for three selected samples from Um Salim-Um Salatit serpentinite.
2h
d
I/I0
ASTM Card
Corresponding mineral phase
d
I/I0
Sample no. 2 12 12.4 24.3 32.5 35.22
7.26 7.37 3.61 2.74 2.53
100 11 68 100 15
7.37 7.1 3.66 2.74 2.52
100 90 95 100 100
Antigorite Clinochrysotile Antigorite Magnesite Antigorite
Sample no. 3 12 12.4 24.3 35.55 43.1
7.26 7.3 3.61 2.52 2.1
100 11 68 15 37
7.37 7.1 3.66 2.52 2.1
100 90 95 100 100
Antigorite Clinochrysotile Antigorite Antigorite Magnesite
Sample no. 4 9.5 12.5 24.3 29.4 30.9 32.5 43.1
9.41 7.15 3.61 3.02 2.89 2.75 2.1
100 11 88 100 100 100 37
9.35 7.1 3.66 3.01 2.89 2.74 2.1
95 90 100 100 100 100 100
Talc Lizardite Antigorite Calcite Dolmite Magnesite Magnesite
Cu radiation, Ni filter, 2h/min.
2.2. Field study
3. Results
A geological map of the study area at scale 1:50,000 was constructed using the Landsat ETM+ images and field observations. Seventy bedrock samples from different rock units were collected for petrographical studies. Fifteen samples were collected from chromite lenses to study the mineralogical features of these deposits.
3.1. Lithological interpretation
2.3. Laboratory work More than forty rock samples representing the different rock units were prepared for thin sections. The petrographic studies were carried out using NIKON polarizing microscope. Twelve rock samples were analyzed for their major and trace element contents. Analyses were carried out in the Central Laboratories of Egyptian Geological Survey by XRF technique using a Philips Pw 1404 spectrometry of the wave length dispersive type for which powder pellets and fusion beads were prepared. Ferrous iron in all samples was determined by wet chemistry. Ten thin-polished sections were prepared and studied using electron microprobe analyses. Electron microprobe analyses of the chromite were carried out at the Wurzburg University, Germany using a CAMECA SV50 electron microprobe under operating conditions of 15 kV accelerating voltage, and 15–16 nA sample current measured on Faraday cage, and 1–2 um beam size; element peaks and backgrounds were each measured over 20 s/synthetic silicate and oxide minerals were used for reference standards. X-ray diffraction technique has been used to identify the serpentinite minerals associated with the chromites ores (Table 1).
Digital processing of Landsat ETM+ images for the study area generated several products ranging from false color composite image (bands 7, 4, 2 in R,G,B) and ratio image (bands 5/ 7, 5/1 and 4 in R, G, B). The Landsat ETM+ false image (bands 7, 4, 2 in R, G, B) for the study area provided an excellent base map for further investigation. Also, the ratioing image (bands 5/7, 5/1, 4) discriminated the different rock types in the area shown in (Fig. 4). The serpentinites indicated by red color, the metavolcanic rocks are characterized by bluish color. The metagabbro rocks indicated by purple color, while the granitic rocks have green colour. 3.2. Geologic setting The serpentinite masses occur in the form of dipping or steeply inclined sheets and lenses, generally, emplaced along thrust faults. They are elongated in the ENE–WSW direction. The area is of moderate to high relief and is made up of fairly massive serpentinites and talc-carbonates. It is tectonically emplaced within a distinct lithologic association of schist, basic metavolcanic rocks and ophiolitic metagabbros (Plate 1a and b). No thermal contact effect of the serpentinites upon the surrounding rocks is observed indicating that they were tectonically emplaced. The serpentinites are characterized by various colours, varying from grayish green to dark green to even black. They are hard, and commonly contain altered bright green antigorite. They are closely jointed, foliated and frequently transformed into talc-carbonates along faults and shear zones (Plate 1c).
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
Figure 4
175
Geological map of G. Um Salim-Um Salatit area (modified after EGSMA map, 1990).
The talc-carbonates are fine to medium-grained, soft with soapy touch and vary in color from pale brown to buff to light cream. Along shear zones feebly schistose chlorite and actinolite schists are usually recognized. Chromite lenses and magnesite veins and pockets occur within the talc-carbonates of G. Um Salim serpentinites (Plate 1d–f).
across the other serpentine minerals. Chromite forms irregular grains of blood red color, shows variable shape skeletal crystal that are corroded by serpentinites. It is slightly replaced by magnetite. The serpentine minerals have been confirmed by X-ray identification according to Whittaker and Zussman (1956).
3.3. Petrography
3.3.2. Dunite serpentinites
The studied serpentinites of Um Salim-Um Salatit include massive serpentinites, dunite serpentinites, harzburgite serpentinites, and subordinate talc carbonates. Further petrographic details are given below. 3.3.1. Massive serpentinites The serpentinites are composed of antigorite, clinochrysotile and lizardite together with minor amounts of carbonate minerals. Petrographically, antigorites are present as flamo-lamellar aggregates or display mesh texture (Plate 2a). Lizardite occurs as very fine-grained crystals or occupies cores of mesh texture, whereas, clinochrysotile occurs as cross-fiber veinlets cutting
These rocks are brown to black in color, coarse-grained, massive and consist of antigorite, lizardite and chrysotile with variable amounts of magnesite, talc, chlorite and tremolite. Olivine relics are also observed in the dunite serpentinites. The antigorites occurs in different forms, commonly as plumes or haphazardly aggregated flakes (Plate 2b). The antigorites are variably replaced by talc. The rock commonly retains a mesh texture with irregular fractures. Bastite occurs as phenoprismatic crystal with schiller structure (Plate 2c). Iddingesite occurs as partially or completely alteration pseudomorphic after olivine crystals. It is pale reddish color, consists of iron oxides in various stage of oxidation and hydration in the dunite serpentinites. Carbonates are present in a fair
176
A.K.A. Salem et al.
Plate 1 (a) General view showing serpentinites (S) and metagabbro rocks in the foreground (MtG). (b) Photograph showing the serpentinites (s) thrusted over the metavolcanic- sedimentary rocks (Mts) at W. Um Salatit. (c) Photograph showing the talc-carbonate in contact with basic schistose metavolcanic at Um Salatit. (d) Photograph showing chromite lens occurs within talc carbonate rocks in Um Salatit area. (e) General view showing serpentinites (s) and metvolcanic rocks (MtV). (f) Photograph showing a small magnesite lens occurs within the serpentinites and related rocks at G. Um Salim.
amount, talc forms a dense matrix of flaky aggregates with random arrangement. Opaques are magnetite and chromite. 3.4. Harzburgite serpentinites These rocks are grayish green in color. Harzburgite serpentinites show relict textures and the original outlines of olivine and orthopyroxene set in a matrix of fibrous antigorite. Bastite is most probably pseudomorphous after enstatite and forms stumpy crystals. Clinochrysotile forms undulatory cross-fiber veinlets cutting through the antigorite matrix, associated with parallel carbonate veins (Plate 2c–e). Talc is formed at the expense of antigorite as scaly aggregates which are often bent. 3.4.1. Talc-carbonate These rocks are associated with serpentinites. Talc carbonates are composed of magnesite, dolomite, calcite and talc. Occasionally, they are highly stained with hematite. The magnesite occurs as irregular and idiomorphic rhombs, as well as clusters of granular aggregates. Dolomite is distinct from magnesite by its characteristic individual rhombohedral habit. It shows subhedral crystals and often has twin lamellae parallel in the short
diagonal as well as to the long diagonal. Calcite is fine to coarse aggregates embedded in the matrix of dolomite, clinochlore and opaques. Chromite occurs as irregular, subhedral and octahedral crystals (Plate 2e). It is rimed by fine minute flakes of chlorite. The talc carbonate bodies enclose chloritetremolite segregations. The presence of the serpentinite minerals was confirmed by X-ray data. 3.5. Geochemistry 3.5.1. Chemical features The results of the major oxide and the trace element contents of the examined samples are given in Table 2. The value of the major oxides are recalculated on water free basis are given in Table 3. CIPW normative values are given in Table 4. 3.6. Petrochemical charactreristics Page (1966) proposed a binary diagram showing the relation between MgO and H2O for the ultramafic rocks (Fig. 5). It is clear from the diagram that the studied serpentinites plot close to the average antigorite. The predominance of antigorite
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
177
Plate 2 (a) Photomicrograph showing mesh textures in the serpentinities. (b) Antigorite occurs in anhedral crystals and aggregates of fibrolamelar structure. (c) Photomicrograph showing vein of chrysotiles cut through antigorites. (d) Photomicrograph showing vein of carbonate minerals cut through antigorite aggregates of fibrolamelar structure. (e) Photomicrograph showing some parallel veins of chrysotiles cut through antigorites. (f) Photomicrograph showing a highly brecciated and fractured chromite eight shot points analyses for chromite were done by electron microprobe at Um Salatit.
over other minerals (lizardite and chrysotile) indicates that the studied serpentinites have suffered prograde metamorphism. The analyzed samples were plotted on the AFM diagram. The analyzed samples fall in the fields of metamorphic peridotites and mafic and ultramafic cumulate ophiolite fields (Fig. 6). These ultramafic cumulates are very similar to the metamorphic peridotites of Coleman (1977). Coleman (1977) advanced two discrimination diagrams based on some major oxides viz. Al2O3 CaO, and MgO (Fig. 7) the ternary diagram Al2O3–CaO- -MgO), all the analyzed samples fall in the metamorphic peridotite field that originated in orogenic belts. The SiO2 % versus FeOt/FeOt + MgO ratio (Fig. 8) is used to discriminate between ultramafic and mafic cumulates. All analyzed samples of the studied serpentinites clearly plot in the ultramafic cumulate field except one sample that plots in the mafic cumulate. The normative composition of the examined serpentinites are plotted on the Ol-Hy-Di diagram Coleman (1977) (Fig. 9). They fall in the harzburgite field. Bucher and Frey (1994) introduced a chemographic projection of CMS–HC system from H2O and CO2 onto SiO2–CaO– MgO plane. The composition of the mantle rocks with anhy-
drous mineralogy OL + OPX + CPX is restricted to the shaded area (labelled M in Fig. 10) defined by forsterite (Ol), enstatite (OPX) and diopside (CPX). The analyzed serpentinites falling on the Fo-En join i.e. they are harzubrgite (depleted mantle peridotites). 3.6.1. Tectonic setting Aumento and Loubat (1971) proposed two diagrams to discriminate between the various types of ultramafic fields, namely low-high temperature Alpine ultramafic rocks, oceanic serpentinites and layered intrusions, based on the amounts of major oxides of SiO2–MgO and Al2O3–CaO. It appears that the studied serpentinites fall in the low temperature Alpine field and are close to the oceanic Mid-Atlantic ridge field (Figs. 11 and 12). Gulacer and Delaloye (1976) proposed plotting Ni/Co versus Ni (Fig. 13). It is clear from the figure that the majority of serpentinites have high Ni/Co ratios and plot within the Alpine type (dunite-periodotite). The high Ni/Co ratio characterizes the ultramafic rocks which are derived from the early stages of magma generation by partial melting of the mantle (Kogarko, 1973).
178 Table 2
A.K.A. Salem et al. Chemical analyses of major oxides and trace elements for Um Salatit-Um Salim serpentinites.
Sample no.
1
2
3
4
5
6
7
8
9
10
11
12
SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5 H2O CO2
39.73 0.01 0.64 6.11 2.08 0.08 38.55 0.15 0.2 0.01 0.02 9.28 0.68
39.78 0.02 0.58 6.78 4.23 0.06 34.72 0.07 0.14 0.03 0.01 8.69 0.59
42.8 0.01 0.7 5.1 1.71 0.093 37.8 0.18 0.05 0.01 0.005 11.79 0.23
38.62 0.01 0.61 7.5 2.28 0.068 38.86 0.08 0.04 0.01 0.008 10.77 0.1
39.85 0.01 0.69 5.99 1.88 0.089 39.87 0.19 0.06 0.01 0.006 10.22 0.24
37.19 0.03 0.39 7.01 3.46 0.06 38.93 0.08 0.18 0.01 0.01 9.17 0.48
38.4 0.01 0.77 7.35 2.22 0.082 38.69 0.59 0.04 0.01 0.006 10.89 0.75
38.21 0.02 0.61 6.64 3.01 0.07 39.42 0.28 0.21 0.02 0.02 8.47 1.04
33.5 0.01 0.47 4.8 1.29 0.113 35.5 2.2 0.05 0.01 0.005 13.26 7.8
38.25 0.01 0.77 5.57 4.43 0.07 37.63 0.04 0.19 0.03 0.01 9.73 0.99
37.82 0.01 0.76 6.39 1.9 0.101 37.06 0.43 0.02 0.01 0.005 14.66 0.55
39.17 0.07 1.49 3.72 6.1 0.05 39.08 0.25 0.26 0.04 0.03 8.43 0.59
Total
97.54
95.7
100.478
98.956
99.105
97
99.808
98.02
99.008
97.72
99.716
99.28
Ba Rb Sr Y Zr Nb Th Pb Ga Zn Cu Ni V Cr Hf Ta Co U W Mo As Bi La Ce
80 3 11 4 38 3 * 4 3 78 7 2662 41 2396 * * 104 * * * * * * *
28 3 9 4 38 5 * 2 2 76 55 3797 913 3018 * * 136 * * * * * * *
17 2 11 2 10 2 3 3 3 45 10 1758 53 2100 3 3 90 5 5 2 4 3 5 5
10 2 9 2 10 3 3 3 3 56 10 1794 46 2646 3 3 119 5 5 2 10 3 5 5
10 2 5 2 10 2 4 4 3 34 10 2054 36 2168 3 3 88 3 5 2 4 3 5 5
12 2 13 3 38 5 * 4 1 55 6 3185 73 1598 * * 205 * * * * * * *
10 2 7 2 8 2 3 3 4 47 10 1593 44 2861 4 3 104 4 5 2 8 3 5 5
18 2 19 4 39 4 * 4 1 60 23 2944 122 2823 * * 150 * * * * * * *
10 2 15 2 9 2 3 3 3 30 10 2052 28 1662 3 3 98 5 5 2 5 3 5 8
44 3 9 4 40 4 * 4 2 70 12 3777 22 3838 * * 138 * * * * * * *
10 2 4 2 11 2 3 3 3 44 10 2110 41 3063 4 3 100 3 7 2 6 3 5 5
16 2 9 7 77 5 * 5 5 63 35 3017 * 1452 * * 145 * * * * * * *
Table 3
Recalculated chemical analyses of major oxides for Um Salatit-Um Salem serpentinites.
Sample no.
1
2
3
4
5
6
7
8
9
10
11
12
SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5
45.36 0.01 0.73 6.98 2.37 0.09 44.02 0.17 0.23 0.01 0.02
46.03 0.02 0.67 7.85 4.89 0.07 40.18 0.08 0.16 0.03 0.01
48.38 0.01 0.79 5.77 1.93 0.11 42.73 0.20 0.06 0.01 0.01
43.84 0.01 0.69 8.51 2.59 0.08 44.12 0.09 0.05 0.01 0.01
44.95 0.01 0.78 6.76 2.12 0.10 44.98 0.21 0.07 0.01 0.01
42.58 0.03 0.45 8.03 3.96 0.07 44.57 0.09 0.21 0.01 0.01
43.55 0.01 0.87 8.34 2.52 0.09 43.88 0.67 0.05 0.01 0.01
43.17 0.02 0.69 7.50 3.40 0.08 44.54 0.32 0.24 0.02 0.02
42.98 0.01 0.60 6.16 1.65 0.14 45.54 2.82 0.06 0.01 0.01
43.97 0.01 0.89 6.40 5.09 0.08 43.25 0.05 0.22 0.03 0.01
44.75 0.01 0.90 7.56 2.25 0.12 43.85 0.51 0.02 0.01 0.01
43.40 0.08 1.65 4.12 6.76 0.06 43.30 0.28 0.29 0.04 0.03
Total
100
100
100
100
100
100
100
100
100
100
100
100
Pearce et al. (1984) postulated a diagram to discriminate between the supra-subduction zone ophiolites) and Mid-Ocean
Ridge basalt ophiolites (MORB) on the basis of Cr–TiO2 content (Fig. 14). The most of the examined samples belong to the
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area Table 4
179
CIPW normative minerals for Um Salatit-Um Salim serpentinites.
Sample no.
1
2
3
4
5
6
7
8
9
10
11
12
Q Or Ab An C(A) Di (wo) Di (en) Di (fs) Hy (en) Hy (fs) Ol (fo) Ol (fa) mt il ap Total OL OPX CPX
0 0.07 1.93 0.72 0.08 0 0 0 25.34 2.53 59.29 6.53 3.44 0.02 0.05 100 59.29 25.34 0
0 0.21 1.37 0.33 0.24 0 0 0 38.04 3.95 43.68 5.01 7.1 0.04 0.03 100 43.68 38.04 0
0 0.07 0.48 0.98 0.33 0 0 0 42.14 3.61 45.28 4.28 2.8 0.02 0.01 100 45.28 42.14 0
0 0.07 0.38 0.4 0.46 0 0 0 21.91 2.7 61.87 8.42 3.75 0.02 0.02 100 61.87 21.91 0
0 0.07 0.57 1.02 0.28 0 0 0 24.51 2.36 61.56 6.53 3.08 0.02 0.01 100 61.56 24.51 0
0 0.07 1.74 0.26 0 0.05 0.04 0 15.89 1.64 66.84 7.63 5.74 0.07 0.02 100 66.84 15.89 0.09
0 0.07 0.38 2.14 0 0.48 0.38 0.05 18.32 2.23 63.7 8.56 3.65 0.02 0.01 100 63.7 18.32 0.86
0 0.13 2.01 0.75 0 0.29 0.23 0.02 16.26 1.61 66.4 7.28 4.93 0.04 0.05 100 66.4 16.26 0.52
0 0.08 0.54 1.32 0 5.28 4.26 0.38 4.43 0.4 73.61 7.26 2.4 0.02 0.01 100 73.61 4.43 9.54
0 0.2 1.85 0.16 0.43 0 0 0 26.3 1.86 57.28 4.48 7.38 0.02 0.03 100 57.28 26.3 0
0 0.07 0.2 2.31 0 0.08 0.06 0.01 24.13 2.69 59.81 7.35 3.26 0.02 0.01 100 59.81 24.13 0.14
0 0.26 2.43 1.18 0.7 0 0 0 26.73 0.48 57.06 1.14 9.8 0.15 0.07 100 57.06 26.73 0
Figure 5 MgO–H2O (wt.%) diagram of the analyzed samples (after Page, 1966).
Figure 7 Triangular diagram of MgO–CaO–Al2O3 (wt.%) for mafic and ultramafic rocks. Komatite field from various sources and MAR represents average composition of Mid-Ocean Ridge basalts. Skaegaard liquid trend is displayed to illustrate possible corollaries to differentiation of a basaltic liquid in ophiolite sequences.
Figure 6 AFM diagram of the examined ultramafic and mafic rocks (after Coleman, 1977).
Figure 8 Variation of SiO2 versus FeOt/FeOt+MgO (wt.%) in the studied ultramafic and mafic rocks (after Coleman, 1977).
180
A.K.A. Salem et al.
Figure 9 Modal composition of the studied in harzburgites and dunites plotted on the Ol-Opx – Cpx diagram for peridotites (after Coleman, 1977).
Figure 10 Chemography of ultramafic rocks CMS-HC system projected from CO2 and H2O on to the plane CaO–MgO–SiO2 showing some rocks and mineral compositions in ultramafic rocks (after Bucher and Frey, 1994).
Figure 11 The analyzed samples plotted on the SiO2–MgO (wt.%) diagram (after Aumento and Loubat, 1971).
Figure 12 Al2O3 and CaO (wt.%) contents in the analyzed samples plotted on the respective diagram (after Aumento and Loubat, 1971).
SSZ ophiolites, i.e., that the examined serpentinite is represented by an ophiolitic mantle sequence which was thrust over the continental margin during the collision stage of back arcbasins. Berhe (1990) considered the Pan African ophiolites as SSZ ophiolites or back arc basin. Baccaluva et al. (1983) proposed a discrimination diagram for some basic and ultramafic rocks on the basis of the ratio Al2O3/TiO2 and TiO2 contents in which Al and Ti are considered to be largely immobile during the alteration processes (Pearce, 1976). The examined samples were plotted on the proposed diagram (Fig. 15), all analyzed samples plot in the harzaburgite and dunite fields with the exception of one sample which falls near the Iherzolite boundary with partial melting range from 10% to 30%, It is worth to mention, that the studied serpentinites were derived from ultramafic rocks (tectonic mantle sequence).
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
181
characteristic bony appearance. It is of good quality. Several veins of magnesite are found in Gabal Um Salim. The host rocks are of moderate to high relief and are made up of fairly massive serpentinites and talc-carbonates. Magnesite veins are quite abundant at high level altitudes especially, at the peak of serpentinite ridge .The magnesite veins are about 1 m. wide and 50 m, long. Sometimes, magnesite occurs as thin veinlets 30–60 cm wide and 1–5 m long. The exploitation of magnesite from Um Salim area is more difficult due to its presence at high altitudes. 4.1. Talc
Figure 13 Ni–Ni/Co ratios of analyzed serpentinite samples from Egypt compared to ratios of Alpine type and Bushveld layered ultramafics (after Gulacer and Delaoye, 1978).
Talc deposits occur at Lat. 25°60 N and Long. 33°510 E .They occur as lenses and sheared pods of variable length and width within the serpentinites and talc-carbonate rocks. Basta and Kamel (1970) studied the mineralogy of Egyptian talc and concluded that the talc of Gabal Um Salim is generally of low grade being rich in tremolite, chlorite and talc carbonate rocks. 4.1.1. Chromite
Figure 14 Cr versus TiO2 (wt.%) for the analyzed serpentinites on the tectonic discrimination diagram (after Pearce et al., 1984).
Chromite is frequently homogeneous and free of any exsolution. It is present either as discrete grains which are completely surrounded by silicate minerals or as chain-like aggregates. The separate chromite crystals are rounded or subhedral occasionally and show ragged boundaries. Skeletal chromite crystals and fine chromite-silicate myrmikite intergrowths are also observed. Chromite in reflected light is gray in color with low reflectivity. However, altered chromite grains display pale gray color with a faint-creamy or pale brownish tint and show higher reflectivity than chromite. Some chromite samples exhibit variable colors due to the presence of fine cracks that cause thin splinter reflection the incident light. These fractures, serve as paths for the transportation of hydrothermal fluids which cause transformation of the chromite grains. These fractures occur as undulated or curved cracks similar to pull-apart texture and are fine-dendritic-like with brecciaed appearance (Plate 2f). 4.1.2. Chemistry of chromite
Figure 15 Diagram showing the genetic relationships (dotted line) between basaltic melts and ultramafic residues divided from a hypothetic pyrolitic source (after Baccaluva et al., 1983).
4. Mineralization Magnesite deposits occur at Lat. 25°100 N and Long. 34°100 E. The magnesite is cryptocrystalline white and massive with
Microprobe analyses of chromite and its structural formulae are calculated from the major oxide contents, on the basis of 32 oxygen (Table 5). On the Fe+3–Cr+3–Al diagram (after Thayer, 1964), the analyzed samples fall in the fields of aluminum chromite (Fig. 16). On the Cr2O3 and total iron diagram, by Thayer (1970) (Fig. 17). It is also observed that most analyzed chromites plot in the Alpine type field except one sample which plots in Blue Ridge field. Many workers believe that the Blue Ridge dunites are pieces of disrupted ophiolites. Moreover, on the Cr+3/ Cr+3+Al versus Fe+2/Fe+2+Mg (Dick and Bullen, 1984) (Fig. 18). It is observed that all the analyzed chromite samples plot in the Alpine field. On the Cr+3–Al–Fe+3 + 2Ti diagram (after Jan and Windley, 1990) (Fig. 19). The analyzed samples also plot in the area of residual peridotite podiform chromatites associated with harzburgite. The low value of TiO2 (up to 0.3%) and lack of significant correlation between titanium and the other elements are considered as distinct features for Alpine type chromite deposits (Dickey,
182 Table 5
A.K.A. Salem et al. Microprobe analyses of Um Salatit chromite.
Sample no.
1
2
3
4
5
6
7
8
SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 MgO CaO MnO FeO NiO ZnO Total FeOt
0.02 0.21 9.81 56.2 3.56 10.21 0.01 0.31 18.7 0.15 0.02 99.2 22.22
0.04 0.13 14.6 55.7 2.89 13.8 0.01 0.36 12.1 0.17 0.04 99.84 14.96
0.02 0.11 13.9 53.89 2.97 15.1 0.02 0.29 13.2 0.19 0.03 99.72 16.14
0.04 0.14 14.7 53.9 3.87 15.3 0.04 0.33 10.98 0.15 0.06 99.51 14.81
0.01 0.1 15.7 53.94 2.96 13.95 0.01 0.39 11.81 0.16 0.05 99.08 14.74
0.02 0.13 15.42 53.7 3.67 15.34 0.02 0.4 11.11 0.12 0.03 99.96 14.74
0.03 0.11 14.61 55.21 3.01 13.78 0.04 0.29 11.94 0.07 0.06 99.15 14.92
0.06 0.12 14.61 53.88 3.71 14.84 0.03 0.37 11.43 0.13 0.02 99.2 15.10
0.01 0.02 4.34 11.12 0.55 5.24 0.00 0.08 2.55 0.03 0.01 23.96 0.33 0.72 0.67 0.05 67.24 71.91 2.56
0.01 0.02 4.15 10.81 0.57 5.75 0.01 0.06 2.79 0.04 0.01 24.21 0.33 0.72 0.67 0.04 67.31 72.23 2.60
0.01 0.03 4.36 10.72 0.73 5.78 0.01 0.07 2.31 0.03 0.01 24.06 0.29 0.71 0.71 0.06 71.50 71.10 2.46
0.00 0.02 4.68 10.78 0.56 5.30 0.00 0.08 2.49 0.03 0.01 23.97 0.32 0.70 0.68 0.05 68.01 69.75 2.31
0.01 0.02 4.54 10.61 0.69 5.76 0.01 0.08 2.32 0.02 0.01 24.05 0.29 0.70 0.71 0.05 71.31 70.03 2.34
0.01 0.02 4.37 11.09 0.57 5.26 0.01 0.06 2.53 0.01 0.01 23.95 0.32 0.72 0.68 0.05 67.50 71.72 2.54
0.02 0.02 4.35 10.78 0.70 5.64 0.01 0.08 2.41 0.03 0.00 24.04 0.30 0.71 0.70 0.06 70.03 71.22 2.47
Cation numbers of ions on basis of 32 (O) Si 0.01 Ti 0.04 Al 3.09 Cr 11.88 Fe+3 0.71 Mg 4.10 Ca 0.00 Mn 0.07 Fe+2 4.17 Ni 0.03 Zn 0.00 Total 24.11 Fe+2/Mg+Fe+2 0.50 Cr/Cr+Al 0.79 Mg/Mg+Fe+2 0.50 0.06 Fe+3/Fe+3+Cr+Al Mg-number 49.57 Cr-number 79.36 Cr/Al 3.84
Cr-number = 100*Cr/(Cr + Al); Mg-number = 100*Mg/(Mg+Fe+2).
Figure 16 Varition of Cr (after Thayer, 1964).
+3
–Al
+3
+3
–Fe
for Um Salatit chromite
1975). The podi-form chromite is characterized by low TiO2 (<0.3) and MnO (<0.27) contents. The presence of podiform chromites in the serpentinites is related typically to SSZ ophiolites (Kroner et al., 1987).
Figure 17 Total FeOt versus Cr2O3 (wt.%) segregated chromite in the Blue Ridge dunite (after Thayer, 1970).
5. Summary and conclusion The serpentinites of Um Salim-Um Salatit build large lenticular bodies located in the Central Eastern Desert of Egypt be-
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
Figure 18 Plots of Cr+3/Cr+3 + Al+3 versus Fe+2/Fe+2 + Mg for the chromite studied (after Dick and Bullen, 1984).
183
Based on geochemical compositions, the examined serpentinites belong to the ultramafic cumulate ophiolite rocks, originally of harzburgite composition (metamorphic peridotite) that represents a fragment of the oceanic lithosphere formed in a back arc basin. The microprobe analyses of chromite show that it belongs to the aluminum chromite type. The podiform chromite is characterized by low TiO2 (<0.3 content) and MnO (<0.27 content) which are considered as distinct features for Alpine type chromite deposits (Dichey, 1975). The chromite is low in Al content in relative to Cr+3 content similar to ophiolitic pediform chromites (Lebance et al., 1980). Podiform chromites in serpentinites are related typically to SSZ ophiolites. Finally, the field and geochemical studies show that the serpentinites belong to the low temperature Alpine type ophiolites. They belong to the ophiolitic mantle sequence formed in SSZ ophiolites which were thrust over the continental margins during the collisional stage of back arc environment. SSZ ophiolites may also form in the initial stages of arc splitting and are also characterized by strongly depleted, harzburgite mantle sequences containing podiform chromite deposits and by crystallization sequences in which olivine is followed by pyroxene (Pearce et al. (1984). Acknowledgements The authors wish to express their gratitude and sincere thanks to Prof. Dr. S. El Gaby for his valuable advices and for critically reading the manuscript. References
Figure 19 Studied chromite plotted on the Cr Fe+3 + 2Ti diagram (after Jan and Windley, 1990).
+3
–Al
+3
–
tween Lat. 25°50 and 25°320 N and Long. 33°520 and 34°020 and extend NE SW trend. They are tectonically emplaced within a distinct lithologic association of schist, basic metavolcanic rocks and ophiolitic metagabbro. Talc deposits occur as lenses and shear pods of variable lengths and widths in the serperntinites and related rocks (talc carbonate). They are formed at the expense of serpentinites along shear zones during a stage of CO2-metasomatism. Most of the magnesite veins were formed during the formation of talc carbonate. Petrographically, the serpentinites are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of magnesite, talc, dolomite, chromite, and chlorite. They show mesh, bastite and knitte textures. The presence of clinochrysotile as cross- feber veinlets traversing the other serpentinite minerals indicates a late crystallization under static conditions as a result of the activity of meteoric waters. The transformation from lizardite and chrysotile to antigorite in serpentenites took place at the beginning of low grade metamorphism (green-schist facies). The examined serpentinites have been derived from harzburgite and dunite due to the prevalence of mesh and bastite texture. Magnesite deposits occur at high levels of the serpentinites outcrop .The chromite lenses present in Wadi Um Salatit serpentinites possess a podiform shape.
Abd El-Rahman, Y., Polat, A., Dilek, Y., Fryer, B., El-Sharkawy, M., Sakran, S., 2009. Geochemistry and tectonic evolution of the Neoproterozoic Wadi Ghadir ophiolite, Eastern Desert. Egypt. Lithos 113, 158–178. Abu El Ela, F.F., Farahat, E.S., 2010. Neoproterozoic podiform chromites in serpentinites of the Abu Meriewa-Hagar Dungash district, Eastern Desert, Egypt: Geotectonic implications and metamorphism. Isl. Arc. 19, 151–164. Akkad, M.K., 1997. On the behaviour of serpentinites and its. Implications. Geol. Surv. paper No. 24. Akaad, M.K., Abu El Ela, A.M., 2002. Geology, of the basement rocks the in Eastern Desert half of the belt between 25o 30o and 26o 30o N central Eastern Desert, Egypt covering parts of sheets NG 36 K2, K3, L1 and NG 36 G5, G6, H4. Geological Survey of Egypt, No. 78. Amstutz, G.C., El-Gaby, S., Ahmed, A.A., Habib, M.E., Khudeir, A.A., 1984. Podiform Chromites in the Rubshi ophiolite association, West of –Quseir, Eastern Desert, Egypt. Bull. Fac. Sci. Assiut. Univ. V 42, 57–93. Aumento, F., Loubat, H., 1971. The Mid-Atlantic ridge near 45°N XVI. Serpentinites ultramafic intrusions. Can. J. Earth. 8, 631–663. Azer, M., Stern, R.J., 2007. Neoproterozoic serpentinites in the Eastern Desert, Egypt: Fragments of fore-arc mantle. Geology 115, 457–472. Baccaluva, l.D., Griolamo, P., Macciotta, G., Morra, V., 1983. Magma affinities and fractionation trends in Ophiolites. Ofioliti. 8, 307–324. Basta, E.Z., Kamel, M., 1970. Mineralogy and ceramic properties of some talcs. Bull. Fac. Sci. Cairo Univer. V 43, 285–299. Berhe, S.M., 1990. Ophiolites in Northeast and East Africa implications for Proterozoic crustal growth. J. Geol. Soc. Lond. 147, 41– 57.
184 Bucher, K., Frey, M., 1994. Petrogenesis of metamorphic rocks 6th Edition complete revision of Winkler’s Textbook. Springer-Verlag, Berlin, Heideberg, New York, London, Paris, Tokyo, Hong Kong, Barcelona, Budapest, 318 pp. Coleman, R.G., 1977. Ophiolites. Springer-Verlag, New York, 229 p. Dick, H.B., Bullen, T., 1984. Chromian spinel as a petrogenetic indicator in abyssal and Alpine type peridotite and spatially associated lavas. Contrib. Mineral. Petrol. 86, 57–76. Dickey Jr., R.J., 1975. A hypothesis of origin for podiform chromite deposits. Geochim. Cosmochim. Acta V 39, 1061–1075. EGSMA, 1990. Basement rocks of Manijam Al Barramiyah. quadrangle Egypt. Scale. 1, 100 000. El Bahariya, G.A., Arai, S., 2003. Petrology and origin of Pan-African serpentinites with particular reference to chromian spinel compositions, Eastern Desert, Egypt: Implication for supra-subduction zone ophiolite. In: 3rd International Conference on the Geology of Africa, pp. 371–388. El-Gaby, S., List, F.K., Tehrani, R., 1988. Geology, evolution and metallogenesis of the Pan-African belt in Egypt. In: El Gaby, S., Greiling, R.O. (Eds.), The Pan-African belt of northeast Africa and adjacent areas. Vieweg Braunschweig, pp. 7–68. El-Gaby, S., List, F.K., Tehrani, R., 1990. The basement complex of the Eastern Desert and Sinai. In: Said, R. (Ed.), The geology of Egypt. A.A Balkema, Rotterdam, Brookfield, pp. 75–184. El Sharkawy, M.A., El Bayoumi, R.M., 1979. The ophiolites of Wadi Ghadir area, Eastern Desert, Egypt. Ann. Geol. Surv. 9, 125–135. Garson, N.S., Shalaby, I.M., 1976. Precambrian lower Paleozoic plate tectonics and metallogenesis in the Red Sea region, Spec. Paper Geol. Ass. Can. V 14, 573–596. Gulacer, O.F., Delaloye, M., 1976. Geochemistry of nickel-cobalt and copper in Alpine ultramafic rocks. Chem. Geol. 17, 269–280. Habib, M.E., Hassan, M.A., Hashad, A.H., Abu Ela, F.F., 1987. Abu Mireiwa ophiolites-melange west Marsa Alam, Egypt Bull. Fac. Sci. Assiut. Univ. V 16, 139–197. Hilmy, M.E., Attia, A.K.A., Boulis, S.N., Ismail, J.SJ., 1991. Mineralogy and geochemistry of chromite ores in Abu Dahr and El-Galala area, Eastern Desert, Egypt. Miner. Soc. Egypt Vol. 3, 1–24. Jan, M.Q., Windley, B.F., 1990. Chromium-spinel silicate chemistry in ultramafic rocks of the Jijal complex. Northwestern Pakistan Journal Petrology 31, 667–715. Khalil, A.E., 1994. Petrological and geochemical studies on serpentinites of Saneyat, El Deigheimi and Muweilih area Qift-Quseir Road, Eastern Desert, Egypt, Ph.D. Thesis Al Azhar Univ., Cairo, Egypt. 1, pp. 1–2. Khalil, A.E.S., Azer, M.K., 2007. Supra-subduction affinity in the Neoproterozoic serpentinites in the Eastern Desert, Egypt:
A.K.A. Salem et al. Evidence from mineral composition. J. Afr. Earth Sci. 49, 136– 152. Khalil, E., Azzaz, S.A., Hussein, E.M., Azb, M.S., 1996. The PanAfrican ophiolites-melange complex and island-arc assemblage of Salim-Salatit area, CDE, Egypt. Delta J. Sci. 20, 140–144. Kogarko, L.N., 1973. The Ni/Co ratio as an indicator of the mantle origin of magmas. Geochem. Miner. Int. 10 (5), 1081–1086. Kroner, A., Greiling, R., Reischman, T., Hussein, I.M., Stern, R.J., Durr, S., Zimmer, M., 1987. Pan-African crustal evolution in the Nubian segment of Northeast Africa. In: Kroner, A. (Ed.), Proterozoic lithosphere Evolution Geodynamic Series. International Lithosphere Program contribution, V.17 American Geophysical Union Washington, 235-25. Lebance, M., Dupuy, C., Cassard, D., Noutte, J., Nicolas, A., 1980. Essaisur la genese des corps podiformes de chromite dans les peridotites ophiolitiques etude des chromite de Nouvelle. In: Panayiotou, A. (Ed.), Ophiolites Proceedings International Symposium Cyprus. Geological Survey Department, Cyprus Publication, pp. 691–701. Page, N.J., 1966. Mineralogy and geochemistry of the serpentine group minerals and srepentinization process, Ph.D. Thesis, Berkeley, Un.W. California. Pearce, J.A., 1976. Statistical analysis of major element patterns in basalts. J. Petrol. V 17, 15–43. Pearce, J.A., Lippard, S.J., Roberts, S., 1984. Characteristic and tectonic significance of supra-subduction zone ophiolites. In: Kokeldar, B.F., Howells, M.F (Eds.), Marginal Basin Geology. Geol. Soc. Spec. Publ. No. 16, pp. 77–94. Salem, A.K.A., Hassan, M.M., Fasfous, B.R., Khalil, A.E., 2004. On chromite mineralization and hosting serpentinites at Gebel Um Khasila area. Bull. NRC Egypt 29 (6), 639–667. Takla, M.A., Noweir, A.M., 1980. Mineralogy and mineral chemistry of ultramafic mass of El-Rubshi, Eastern Desert, Egypt. Jb. Miner. Abh. 140, 17–28. Takla, M.A., Noweir, A.M., Aly, S.A., 1975. Ore mineralogy of the serpentinites of Bir El-Kubbaniya. Um Khors area, Egypt Chem. Erde. 34, 244–250. Thayer, T.P., 1964. Principle features as origin podiform chromite deposits and some observations in the Guleman-Soridag district, Turkey. Econ. Geol. 59, 1497–1524. Thayer, M.A., 1970. Chromite segregation as petrogenic indicator, Geol. Soc. of Africa. Symposium on the Bushveld Igneous Complex and other layered intrusions. Spec. Publ. 1, 380–390. Whittaker, F., Zussman, J., 1956. The characterization of serpentine minerals by X-ray diffraction. Miner. Mag. 31, 107.
National Authority for Remote Sensing and Space Sciences
The Egyptian Journal of Remote Sensing and Space Sciences www.elsevier.com/locate/ejrs www.sciencedirect.com
Research Paper
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area, Central Eastern Desert, Egypt A.K.A. Salem a, A.E. Khalil a, T.M. Ramadan a b
b,*
Department of Geological Sciences, National Research Centre, Egypt The National Authority for Remote Sensing and Space Sciences, Cairo, Egypt
Received 17 November 2011; accepted 30 November 2011 Available online 20 December 2012
KEYWORDS Tectonic setting; Pan-African; Serpentinites; Podiform chromite; Subduction zone; Ultramafic cumulate
Abstract This research aims at integrating remote sensing data and field studies for lithological mapping of different rock units. Geological interpretation of Landsat ETM+ image and field studies revealed that the study area is mainly covered by ophiolitic ultramafic rocks, ophiolitic metgabbros, schistose metavolocanics and alkaline granites. The serpentinites masses occur in the form of dipping or steeply inclined sheets or lenses generally emplaced along thrust faults .They are elongated in a ENE-WSW direction. They are composed essentially of antigorite, clinochrysotile and lizardite with subordinate amount of magnesite, chromite, magnetite, talc and chlorite. Geochemically, the studied serpentinites are rich in MgO, Cr, Ni and high Ni/Co ratio. The examined serpentinites belong to the ultramafic cumulate ophiolite rocks and it was originally a harzburgite that represents a fragment of the oceanic lithosphere formed in a back arc basin, i.e., they belong to an ophiolitic mantle sequence which was formed above subduction zone (SSZ). Several mineral deposits are associated with the ophiolitic ultramafic rocks such as magnesite, talc and chromite. Chromite lenses are mainly encountered at wadi level in a podiform shape. Microprobe analyses of chromite show that the chromite is an aluminum chromite type. The podiform chromite is characterized by low TiO2 (<0.3%) and MnO (<0.27% contents) which are considered as distinct feature of Alpine type chromite deposits. The presence of podiform chromites in
* Corresponding author. E-mail address: [email protected] (T.M. Ramadan). q Peer review under responsibility of National Authority for Remote Sensing and Space Sciences
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172
A.K.A. Salem et al. the serpentinites are related typically to supra subduction zone ophiolites (SSZ). The studied serpentinites show similarities to the Alpine type serpentinites. Ó 2011 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. All rights reserved.
1. Introduction Serpentinites are widely distributed in the ophiolite assemblage that constitutes an important part of the basement complex of the Eastern Desert of Egypt (Takla et al., 1975; Garson and Shalaby, 1976; El Sharkawy and El Bayoumy, 1979; Takla and Noweir, 1980; Amstutz et al.,1984; Habib et al.,1987; El Gaby et al., 1988, 1990; Hilmy et al., 1991; Khalil, 1994 ; Khalil et al., 1996; Akaad, 1997; Akaad and Abu Ella, 2002; El Bahariya and Arai, 2003; Salem et al., 2004; Azer and Stern, 2007; Khalil and Azer, 2007; Abd El-Rahman et al., 2009; Abu El Ela and Farahat, 2010). Um Salim-Um Salatit area is located in the Central Eastern Desert between Lat. 25°50 –25°320 N and Long. 33°520 –34°020 E (Fig. 1). The present paper deals with detailed geological, mineralogical and geochemical studies of the ultramafic rocks exposed in the study area.
Figure 1
These serpentinites form huge lenticular bodies with ENE– WSW trend, conformable with the regional strike of the host rocks. 2. Methodology 2.1. Remote sensing data In this study, one scene of Enhanced Landsat ETM+ data (Path 174 and Raw 42, acquired on March, 2000) covering the investigated area, has been geometrically corrected and radiometrically balanced using the ERDAS imagine 9.1 at the National Authority for Remote Sensing and Space Sciences (NARSS), Cairo. The applied techniques include false color composite image (bands 7, 4, 3, Fig. 2) to produce structural map and rationing image (bands 5/7, 5/1, 4, Fig. 3) to distinguish the different rock units.
Landsat ETM+ image showing the location of study area (white box).
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
Figure 2
Figure 3
False colour Landsat ETM+ image (bands 7, 4, 3 in R, G, B) for the study area.
Landsat ETM+ ratio image (bands 5/7, 5/1, 4 in R, G, B) for the study area.
173
174 Table 1
A.K.A. Salem et al. X-ray diffraction data for three selected samples from Um Salim-Um Salatit serpentinite.
2h
d
I/I0
ASTM Card
Corresponding mineral phase
d
I/I0
Sample no. 2 12 12.4 24.3 32.5 35.22
7.26 7.37 3.61 2.74 2.53
100 11 68 100 15
7.37 7.1 3.66 2.74 2.52
100 90 95 100 100
Antigorite Clinochrysotile Antigorite Magnesite Antigorite
Sample no. 3 12 12.4 24.3 35.55 43.1
7.26 7.3 3.61 2.52 2.1
100 11 68 15 37
7.37 7.1 3.66 2.52 2.1
100 90 95 100 100
Antigorite Clinochrysotile Antigorite Antigorite Magnesite
Sample no. 4 9.5 12.5 24.3 29.4 30.9 32.5 43.1
9.41 7.15 3.61 3.02 2.89 2.75 2.1
100 11 88 100 100 100 37
9.35 7.1 3.66 3.01 2.89 2.74 2.1
95 90 100 100 100 100 100
Talc Lizardite Antigorite Calcite Dolmite Magnesite Magnesite
Cu radiation, Ni filter, 2h/min.
2.2. Field study
3. Results
A geological map of the study area at scale 1:50,000 was constructed using the Landsat ETM+ images and field observations. Seventy bedrock samples from different rock units were collected for petrographical studies. Fifteen samples were collected from chromite lenses to study the mineralogical features of these deposits.
3.1. Lithological interpretation
2.3. Laboratory work More than forty rock samples representing the different rock units were prepared for thin sections. The petrographic studies were carried out using NIKON polarizing microscope. Twelve rock samples were analyzed for their major and trace element contents. Analyses were carried out in the Central Laboratories of Egyptian Geological Survey by XRF technique using a Philips Pw 1404 spectrometry of the wave length dispersive type for which powder pellets and fusion beads were prepared. Ferrous iron in all samples was determined by wet chemistry. Ten thin-polished sections were prepared and studied using electron microprobe analyses. Electron microprobe analyses of the chromite were carried out at the Wurzburg University, Germany using a CAMECA SV50 electron microprobe under operating conditions of 15 kV accelerating voltage, and 15–16 nA sample current measured on Faraday cage, and 1–2 um beam size; element peaks and backgrounds were each measured over 20 s/synthetic silicate and oxide minerals were used for reference standards. X-ray diffraction technique has been used to identify the serpentinite minerals associated with the chromites ores (Table 1).
Digital processing of Landsat ETM+ images for the study area generated several products ranging from false color composite image (bands 7, 4, 2 in R,G,B) and ratio image (bands 5/ 7, 5/1 and 4 in R, G, B). The Landsat ETM+ false image (bands 7, 4, 2 in R, G, B) for the study area provided an excellent base map for further investigation. Also, the ratioing image (bands 5/7, 5/1, 4) discriminated the different rock types in the area shown in (Fig. 4). The serpentinites indicated by red color, the metavolcanic rocks are characterized by bluish color. The metagabbro rocks indicated by purple color, while the granitic rocks have green colour. 3.2. Geologic setting The serpentinite masses occur in the form of dipping or steeply inclined sheets and lenses, generally, emplaced along thrust faults. They are elongated in the ENE–WSW direction. The area is of moderate to high relief and is made up of fairly massive serpentinites and talc-carbonates. It is tectonically emplaced within a distinct lithologic association of schist, basic metavolcanic rocks and ophiolitic metagabbros (Plate 1a and b). No thermal contact effect of the serpentinites upon the surrounding rocks is observed indicating that they were tectonically emplaced. The serpentinites are characterized by various colours, varying from grayish green to dark green to even black. They are hard, and commonly contain altered bright green antigorite. They are closely jointed, foliated and frequently transformed into talc-carbonates along faults and shear zones (Plate 1c).
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
Figure 4
175
Geological map of G. Um Salim-Um Salatit area (modified after EGSMA map, 1990).
The talc-carbonates are fine to medium-grained, soft with soapy touch and vary in color from pale brown to buff to light cream. Along shear zones feebly schistose chlorite and actinolite schists are usually recognized. Chromite lenses and magnesite veins and pockets occur within the talc-carbonates of G. Um Salim serpentinites (Plate 1d–f).
across the other serpentine minerals. Chromite forms irregular grains of blood red color, shows variable shape skeletal crystal that are corroded by serpentinites. It is slightly replaced by magnetite. The serpentine minerals have been confirmed by X-ray identification according to Whittaker and Zussman (1956).
3.3. Petrography
3.3.2. Dunite serpentinites
The studied serpentinites of Um Salim-Um Salatit include massive serpentinites, dunite serpentinites, harzburgite serpentinites, and subordinate talc carbonates. Further petrographic details are given below. 3.3.1. Massive serpentinites The serpentinites are composed of antigorite, clinochrysotile and lizardite together with minor amounts of carbonate minerals. Petrographically, antigorites are present as flamo-lamellar aggregates or display mesh texture (Plate 2a). Lizardite occurs as very fine-grained crystals or occupies cores of mesh texture, whereas, clinochrysotile occurs as cross-fiber veinlets cutting
These rocks are brown to black in color, coarse-grained, massive and consist of antigorite, lizardite and chrysotile with variable amounts of magnesite, talc, chlorite and tremolite. Olivine relics are also observed in the dunite serpentinites. The antigorites occurs in different forms, commonly as plumes or haphazardly aggregated flakes (Plate 2b). The antigorites are variably replaced by talc. The rock commonly retains a mesh texture with irregular fractures. Bastite occurs as phenoprismatic crystal with schiller structure (Plate 2c). Iddingesite occurs as partially or completely alteration pseudomorphic after olivine crystals. It is pale reddish color, consists of iron oxides in various stage of oxidation and hydration in the dunite serpentinites. Carbonates are present in a fair
176
A.K.A. Salem et al.
Plate 1 (a) General view showing serpentinites (S) and metagabbro rocks in the foreground (MtG). (b) Photograph showing the serpentinites (s) thrusted over the metavolcanic- sedimentary rocks (Mts) at W. Um Salatit. (c) Photograph showing the talc-carbonate in contact with basic schistose metavolcanic at Um Salatit. (d) Photograph showing chromite lens occurs within talc carbonate rocks in Um Salatit area. (e) General view showing serpentinites (s) and metvolcanic rocks (MtV). (f) Photograph showing a small magnesite lens occurs within the serpentinites and related rocks at G. Um Salim.
amount, talc forms a dense matrix of flaky aggregates with random arrangement. Opaques are magnetite and chromite. 3.4. Harzburgite serpentinites These rocks are grayish green in color. Harzburgite serpentinites show relict textures and the original outlines of olivine and orthopyroxene set in a matrix of fibrous antigorite. Bastite is most probably pseudomorphous after enstatite and forms stumpy crystals. Clinochrysotile forms undulatory cross-fiber veinlets cutting through the antigorite matrix, associated with parallel carbonate veins (Plate 2c–e). Talc is formed at the expense of antigorite as scaly aggregates which are often bent. 3.4.1. Talc-carbonate These rocks are associated with serpentinites. Talc carbonates are composed of magnesite, dolomite, calcite and talc. Occasionally, they are highly stained with hematite. The magnesite occurs as irregular and idiomorphic rhombs, as well as clusters of granular aggregates. Dolomite is distinct from magnesite by its characteristic individual rhombohedral habit. It shows subhedral crystals and often has twin lamellae parallel in the short
diagonal as well as to the long diagonal. Calcite is fine to coarse aggregates embedded in the matrix of dolomite, clinochlore and opaques. Chromite occurs as irregular, subhedral and octahedral crystals (Plate 2e). It is rimed by fine minute flakes of chlorite. The talc carbonate bodies enclose chloritetremolite segregations. The presence of the serpentinite minerals was confirmed by X-ray data. 3.5. Geochemistry 3.5.1. Chemical features The results of the major oxide and the trace element contents of the examined samples are given in Table 2. The value of the major oxides are recalculated on water free basis are given in Table 3. CIPW normative values are given in Table 4. 3.6. Petrochemical charactreristics Page (1966) proposed a binary diagram showing the relation between MgO and H2O for the ultramafic rocks (Fig. 5). It is clear from the diagram that the studied serpentinites plot close to the average antigorite. The predominance of antigorite
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
177
Plate 2 (a) Photomicrograph showing mesh textures in the serpentinities. (b) Antigorite occurs in anhedral crystals and aggregates of fibrolamelar structure. (c) Photomicrograph showing vein of chrysotiles cut through antigorites. (d) Photomicrograph showing vein of carbonate minerals cut through antigorite aggregates of fibrolamelar structure. (e) Photomicrograph showing some parallel veins of chrysotiles cut through antigorites. (f) Photomicrograph showing a highly brecciated and fractured chromite eight shot points analyses for chromite were done by electron microprobe at Um Salatit.
over other minerals (lizardite and chrysotile) indicates that the studied serpentinites have suffered prograde metamorphism. The analyzed samples were plotted on the AFM diagram. The analyzed samples fall in the fields of metamorphic peridotites and mafic and ultramafic cumulate ophiolite fields (Fig. 6). These ultramafic cumulates are very similar to the metamorphic peridotites of Coleman (1977). Coleman (1977) advanced two discrimination diagrams based on some major oxides viz. Al2O3 CaO, and MgO (Fig. 7) the ternary diagram Al2O3–CaO- -MgO), all the analyzed samples fall in the metamorphic peridotite field that originated in orogenic belts. The SiO2 % versus FeOt/FeOt + MgO ratio (Fig. 8) is used to discriminate between ultramafic and mafic cumulates. All analyzed samples of the studied serpentinites clearly plot in the ultramafic cumulate field except one sample that plots in the mafic cumulate. The normative composition of the examined serpentinites are plotted on the Ol-Hy-Di diagram Coleman (1977) (Fig. 9). They fall in the harzburgite field. Bucher and Frey (1994) introduced a chemographic projection of CMS–HC system from H2O and CO2 onto SiO2–CaO– MgO plane. The composition of the mantle rocks with anhy-
drous mineralogy OL + OPX + CPX is restricted to the shaded area (labelled M in Fig. 10) defined by forsterite (Ol), enstatite (OPX) and diopside (CPX). The analyzed serpentinites falling on the Fo-En join i.e. they are harzubrgite (depleted mantle peridotites). 3.6.1. Tectonic setting Aumento and Loubat (1971) proposed two diagrams to discriminate between the various types of ultramafic fields, namely low-high temperature Alpine ultramafic rocks, oceanic serpentinites and layered intrusions, based on the amounts of major oxides of SiO2–MgO and Al2O3–CaO. It appears that the studied serpentinites fall in the low temperature Alpine field and are close to the oceanic Mid-Atlantic ridge field (Figs. 11 and 12). Gulacer and Delaloye (1976) proposed plotting Ni/Co versus Ni (Fig. 13). It is clear from the figure that the majority of serpentinites have high Ni/Co ratios and plot within the Alpine type (dunite-periodotite). The high Ni/Co ratio characterizes the ultramafic rocks which are derived from the early stages of magma generation by partial melting of the mantle (Kogarko, 1973).
178 Table 2
A.K.A. Salem et al. Chemical analyses of major oxides and trace elements for Um Salatit-Um Salim serpentinites.
Sample no.
1
2
3
4
5
6
7
8
9
10
11
12
SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5 H2O CO2
39.73 0.01 0.64 6.11 2.08 0.08 38.55 0.15 0.2 0.01 0.02 9.28 0.68
39.78 0.02 0.58 6.78 4.23 0.06 34.72 0.07 0.14 0.03 0.01 8.69 0.59
42.8 0.01 0.7 5.1 1.71 0.093 37.8 0.18 0.05 0.01 0.005 11.79 0.23
38.62 0.01 0.61 7.5 2.28 0.068 38.86 0.08 0.04 0.01 0.008 10.77 0.1
39.85 0.01 0.69 5.99 1.88 0.089 39.87 0.19 0.06 0.01 0.006 10.22 0.24
37.19 0.03 0.39 7.01 3.46 0.06 38.93 0.08 0.18 0.01 0.01 9.17 0.48
38.4 0.01 0.77 7.35 2.22 0.082 38.69 0.59 0.04 0.01 0.006 10.89 0.75
38.21 0.02 0.61 6.64 3.01 0.07 39.42 0.28 0.21 0.02 0.02 8.47 1.04
33.5 0.01 0.47 4.8 1.29 0.113 35.5 2.2 0.05 0.01 0.005 13.26 7.8
38.25 0.01 0.77 5.57 4.43 0.07 37.63 0.04 0.19 0.03 0.01 9.73 0.99
37.82 0.01 0.76 6.39 1.9 0.101 37.06 0.43 0.02 0.01 0.005 14.66 0.55
39.17 0.07 1.49 3.72 6.1 0.05 39.08 0.25 0.26 0.04 0.03 8.43 0.59
Total
97.54
95.7
100.478
98.956
99.105
97
99.808
98.02
99.008
97.72
99.716
99.28
Ba Rb Sr Y Zr Nb Th Pb Ga Zn Cu Ni V Cr Hf Ta Co U W Mo As Bi La Ce
80 3 11 4 38 3 * 4 3 78 7 2662 41 2396 * * 104 * * * * * * *
28 3 9 4 38 5 * 2 2 76 55 3797 913 3018 * * 136 * * * * * * *
17 2 11 2 10 2 3 3 3 45 10 1758 53 2100 3 3 90 5 5 2 4 3 5 5
10 2 9 2 10 3 3 3 3 56 10 1794 46 2646 3 3 119 5 5 2 10 3 5 5
10 2 5 2 10 2 4 4 3 34 10 2054 36 2168 3 3 88 3 5 2 4 3 5 5
12 2 13 3 38 5 * 4 1 55 6 3185 73 1598 * * 205 * * * * * * *
10 2 7 2 8 2 3 3 4 47 10 1593 44 2861 4 3 104 4 5 2 8 3 5 5
18 2 19 4 39 4 * 4 1 60 23 2944 122 2823 * * 150 * * * * * * *
10 2 15 2 9 2 3 3 3 30 10 2052 28 1662 3 3 98 5 5 2 5 3 5 8
44 3 9 4 40 4 * 4 2 70 12 3777 22 3838 * * 138 * * * * * * *
10 2 4 2 11 2 3 3 3 44 10 2110 41 3063 4 3 100 3 7 2 6 3 5 5
16 2 9 7 77 5 * 5 5 63 35 3017 * 1452 * * 145 * * * * * * *
Table 3
Recalculated chemical analyses of major oxides for Um Salatit-Um Salem serpentinites.
Sample no.
1
2
3
4
5
6
7
8
9
10
11
12
SiO2 TiO2 Al2O3 FeO Fe2O3 MnO MgO CaO Na2O K2O P2O5
45.36 0.01 0.73 6.98 2.37 0.09 44.02 0.17 0.23 0.01 0.02
46.03 0.02 0.67 7.85 4.89 0.07 40.18 0.08 0.16 0.03 0.01
48.38 0.01 0.79 5.77 1.93 0.11 42.73 0.20 0.06 0.01 0.01
43.84 0.01 0.69 8.51 2.59 0.08 44.12 0.09 0.05 0.01 0.01
44.95 0.01 0.78 6.76 2.12 0.10 44.98 0.21 0.07 0.01 0.01
42.58 0.03 0.45 8.03 3.96 0.07 44.57 0.09 0.21 0.01 0.01
43.55 0.01 0.87 8.34 2.52 0.09 43.88 0.67 0.05 0.01 0.01
43.17 0.02 0.69 7.50 3.40 0.08 44.54 0.32 0.24 0.02 0.02
42.98 0.01 0.60 6.16 1.65 0.14 45.54 2.82 0.06 0.01 0.01
43.97 0.01 0.89 6.40 5.09 0.08 43.25 0.05 0.22 0.03 0.01
44.75 0.01 0.90 7.56 2.25 0.12 43.85 0.51 0.02 0.01 0.01
43.40 0.08 1.65 4.12 6.76 0.06 43.30 0.28 0.29 0.04 0.03
Total
100
100
100
100
100
100
100
100
100
100
100
100
Pearce et al. (1984) postulated a diagram to discriminate between the supra-subduction zone ophiolites) and Mid-Ocean
Ridge basalt ophiolites (MORB) on the basis of Cr–TiO2 content (Fig. 14). The most of the examined samples belong to the
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area Table 4
179
CIPW normative minerals for Um Salatit-Um Salim serpentinites.
Sample no.
1
2
3
4
5
6
7
8
9
10
11
12
Q Or Ab An C(A) Di (wo) Di (en) Di (fs) Hy (en) Hy (fs) Ol (fo) Ol (fa) mt il ap Total OL OPX CPX
0 0.07 1.93 0.72 0.08 0 0 0 25.34 2.53 59.29 6.53 3.44 0.02 0.05 100 59.29 25.34 0
0 0.21 1.37 0.33 0.24 0 0 0 38.04 3.95 43.68 5.01 7.1 0.04 0.03 100 43.68 38.04 0
0 0.07 0.48 0.98 0.33 0 0 0 42.14 3.61 45.28 4.28 2.8 0.02 0.01 100 45.28 42.14 0
0 0.07 0.38 0.4 0.46 0 0 0 21.91 2.7 61.87 8.42 3.75 0.02 0.02 100 61.87 21.91 0
0 0.07 0.57 1.02 0.28 0 0 0 24.51 2.36 61.56 6.53 3.08 0.02 0.01 100 61.56 24.51 0
0 0.07 1.74 0.26 0 0.05 0.04 0 15.89 1.64 66.84 7.63 5.74 0.07 0.02 100 66.84 15.89 0.09
0 0.07 0.38 2.14 0 0.48 0.38 0.05 18.32 2.23 63.7 8.56 3.65 0.02 0.01 100 63.7 18.32 0.86
0 0.13 2.01 0.75 0 0.29 0.23 0.02 16.26 1.61 66.4 7.28 4.93 0.04 0.05 100 66.4 16.26 0.52
0 0.08 0.54 1.32 0 5.28 4.26 0.38 4.43 0.4 73.61 7.26 2.4 0.02 0.01 100 73.61 4.43 9.54
0 0.2 1.85 0.16 0.43 0 0 0 26.3 1.86 57.28 4.48 7.38 0.02 0.03 100 57.28 26.3 0
0 0.07 0.2 2.31 0 0.08 0.06 0.01 24.13 2.69 59.81 7.35 3.26 0.02 0.01 100 59.81 24.13 0.14
0 0.26 2.43 1.18 0.7 0 0 0 26.73 0.48 57.06 1.14 9.8 0.15 0.07 100 57.06 26.73 0
Figure 5 MgO–H2O (wt.%) diagram of the analyzed samples (after Page, 1966).
Figure 7 Triangular diagram of MgO–CaO–Al2O3 (wt.%) for mafic and ultramafic rocks. Komatite field from various sources and MAR represents average composition of Mid-Ocean Ridge basalts. Skaegaard liquid trend is displayed to illustrate possible corollaries to differentiation of a basaltic liquid in ophiolite sequences.
Figure 6 AFM diagram of the examined ultramafic and mafic rocks (after Coleman, 1977).
Figure 8 Variation of SiO2 versus FeOt/FeOt+MgO (wt.%) in the studied ultramafic and mafic rocks (after Coleman, 1977).
180
A.K.A. Salem et al.
Figure 9 Modal composition of the studied in harzburgites and dunites plotted on the Ol-Opx – Cpx diagram for peridotites (after Coleman, 1977).
Figure 10 Chemography of ultramafic rocks CMS-HC system projected from CO2 and H2O on to the plane CaO–MgO–SiO2 showing some rocks and mineral compositions in ultramafic rocks (after Bucher and Frey, 1994).
Figure 11 The analyzed samples plotted on the SiO2–MgO (wt.%) diagram (after Aumento and Loubat, 1971).
Figure 12 Al2O3 and CaO (wt.%) contents in the analyzed samples plotted on the respective diagram (after Aumento and Loubat, 1971).
SSZ ophiolites, i.e., that the examined serpentinite is represented by an ophiolitic mantle sequence which was thrust over the continental margin during the collision stage of back arcbasins. Berhe (1990) considered the Pan African ophiolites as SSZ ophiolites or back arc basin. Baccaluva et al. (1983) proposed a discrimination diagram for some basic and ultramafic rocks on the basis of the ratio Al2O3/TiO2 and TiO2 contents in which Al and Ti are considered to be largely immobile during the alteration processes (Pearce, 1976). The examined samples were plotted on the proposed diagram (Fig. 15), all analyzed samples plot in the harzaburgite and dunite fields with the exception of one sample which falls near the Iherzolite boundary with partial melting range from 10% to 30%, It is worth to mention, that the studied serpentinites were derived from ultramafic rocks (tectonic mantle sequence).
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
181
characteristic bony appearance. It is of good quality. Several veins of magnesite are found in Gabal Um Salim. The host rocks are of moderate to high relief and are made up of fairly massive serpentinites and talc-carbonates. Magnesite veins are quite abundant at high level altitudes especially, at the peak of serpentinite ridge .The magnesite veins are about 1 m. wide and 50 m, long. Sometimes, magnesite occurs as thin veinlets 30–60 cm wide and 1–5 m long. The exploitation of magnesite from Um Salim area is more difficult due to its presence at high altitudes. 4.1. Talc
Figure 13 Ni–Ni/Co ratios of analyzed serpentinite samples from Egypt compared to ratios of Alpine type and Bushveld layered ultramafics (after Gulacer and Delaoye, 1978).
Talc deposits occur at Lat. 25°60 N and Long. 33°510 E .They occur as lenses and sheared pods of variable length and width within the serpentinites and talc-carbonate rocks. Basta and Kamel (1970) studied the mineralogy of Egyptian talc and concluded that the talc of Gabal Um Salim is generally of low grade being rich in tremolite, chlorite and talc carbonate rocks. 4.1.1. Chromite
Figure 14 Cr versus TiO2 (wt.%) for the analyzed serpentinites on the tectonic discrimination diagram (after Pearce et al., 1984).
Chromite is frequently homogeneous and free of any exsolution. It is present either as discrete grains which are completely surrounded by silicate minerals or as chain-like aggregates. The separate chromite crystals are rounded or subhedral occasionally and show ragged boundaries. Skeletal chromite crystals and fine chromite-silicate myrmikite intergrowths are also observed. Chromite in reflected light is gray in color with low reflectivity. However, altered chromite grains display pale gray color with a faint-creamy or pale brownish tint and show higher reflectivity than chromite. Some chromite samples exhibit variable colors due to the presence of fine cracks that cause thin splinter reflection the incident light. These fractures, serve as paths for the transportation of hydrothermal fluids which cause transformation of the chromite grains. These fractures occur as undulated or curved cracks similar to pull-apart texture and are fine-dendritic-like with brecciaed appearance (Plate 2f). 4.1.2. Chemistry of chromite
Figure 15 Diagram showing the genetic relationships (dotted line) between basaltic melts and ultramafic residues divided from a hypothetic pyrolitic source (after Baccaluva et al., 1983).
4. Mineralization Magnesite deposits occur at Lat. 25°100 N and Long. 34°100 E. The magnesite is cryptocrystalline white and massive with
Microprobe analyses of chromite and its structural formulae are calculated from the major oxide contents, on the basis of 32 oxygen (Table 5). On the Fe+3–Cr+3–Al diagram (after Thayer, 1964), the analyzed samples fall in the fields of aluminum chromite (Fig. 16). On the Cr2O3 and total iron diagram, by Thayer (1970) (Fig. 17). It is also observed that most analyzed chromites plot in the Alpine type field except one sample which plots in Blue Ridge field. Many workers believe that the Blue Ridge dunites are pieces of disrupted ophiolites. Moreover, on the Cr+3/ Cr+3+Al versus Fe+2/Fe+2+Mg (Dick and Bullen, 1984) (Fig. 18). It is observed that all the analyzed chromite samples plot in the Alpine field. On the Cr+3–Al–Fe+3 + 2Ti diagram (after Jan and Windley, 1990) (Fig. 19). The analyzed samples also plot in the area of residual peridotite podiform chromatites associated with harzburgite. The low value of TiO2 (up to 0.3%) and lack of significant correlation between titanium and the other elements are considered as distinct features for Alpine type chromite deposits (Dickey,
182 Table 5
A.K.A. Salem et al. Microprobe analyses of Um Salatit chromite.
Sample no.
1
2
3
4
5
6
7
8
SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 MgO CaO MnO FeO NiO ZnO Total FeOt
0.02 0.21 9.81 56.2 3.56 10.21 0.01 0.31 18.7 0.15 0.02 99.2 22.22
0.04 0.13 14.6 55.7 2.89 13.8 0.01 0.36 12.1 0.17 0.04 99.84 14.96
0.02 0.11 13.9 53.89 2.97 15.1 0.02 0.29 13.2 0.19 0.03 99.72 16.14
0.04 0.14 14.7 53.9 3.87 15.3 0.04 0.33 10.98 0.15 0.06 99.51 14.81
0.01 0.1 15.7 53.94 2.96 13.95 0.01 0.39 11.81 0.16 0.05 99.08 14.74
0.02 0.13 15.42 53.7 3.67 15.34 0.02 0.4 11.11 0.12 0.03 99.96 14.74
0.03 0.11 14.61 55.21 3.01 13.78 0.04 0.29 11.94 0.07 0.06 99.15 14.92
0.06 0.12 14.61 53.88 3.71 14.84 0.03 0.37 11.43 0.13 0.02 99.2 15.10
0.01 0.02 4.34 11.12 0.55 5.24 0.00 0.08 2.55 0.03 0.01 23.96 0.33 0.72 0.67 0.05 67.24 71.91 2.56
0.01 0.02 4.15 10.81 0.57 5.75 0.01 0.06 2.79 0.04 0.01 24.21 0.33 0.72 0.67 0.04 67.31 72.23 2.60
0.01 0.03 4.36 10.72 0.73 5.78 0.01 0.07 2.31 0.03 0.01 24.06 0.29 0.71 0.71 0.06 71.50 71.10 2.46
0.00 0.02 4.68 10.78 0.56 5.30 0.00 0.08 2.49 0.03 0.01 23.97 0.32 0.70 0.68 0.05 68.01 69.75 2.31
0.01 0.02 4.54 10.61 0.69 5.76 0.01 0.08 2.32 0.02 0.01 24.05 0.29 0.70 0.71 0.05 71.31 70.03 2.34
0.01 0.02 4.37 11.09 0.57 5.26 0.01 0.06 2.53 0.01 0.01 23.95 0.32 0.72 0.68 0.05 67.50 71.72 2.54
0.02 0.02 4.35 10.78 0.70 5.64 0.01 0.08 2.41 0.03 0.00 24.04 0.30 0.71 0.70 0.06 70.03 71.22 2.47
Cation numbers of ions on basis of 32 (O) Si 0.01 Ti 0.04 Al 3.09 Cr 11.88 Fe+3 0.71 Mg 4.10 Ca 0.00 Mn 0.07 Fe+2 4.17 Ni 0.03 Zn 0.00 Total 24.11 Fe+2/Mg+Fe+2 0.50 Cr/Cr+Al 0.79 Mg/Mg+Fe+2 0.50 0.06 Fe+3/Fe+3+Cr+Al Mg-number 49.57 Cr-number 79.36 Cr/Al 3.84
Cr-number = 100*Cr/(Cr + Al); Mg-number = 100*Mg/(Mg+Fe+2).
Figure 16 Varition of Cr (after Thayer, 1964).
+3
–Al
+3
+3
–Fe
for Um Salatit chromite
1975). The podi-form chromite is characterized by low TiO2 (<0.3) and MnO (<0.27) contents. The presence of podiform chromites in the serpentinites is related typically to SSZ ophiolites (Kroner et al., 1987).
Figure 17 Total FeOt versus Cr2O3 (wt.%) segregated chromite in the Blue Ridge dunite (after Thayer, 1970).
5. Summary and conclusion The serpentinites of Um Salim-Um Salatit build large lenticular bodies located in the Central Eastern Desert of Egypt be-
Geology, geochemistry and tectonic setting of Pan-African serpentinites of Um Salim-Um Salatit area
Figure 18 Plots of Cr+3/Cr+3 + Al+3 versus Fe+2/Fe+2 + Mg for the chromite studied (after Dick and Bullen, 1984).
183
Based on geochemical compositions, the examined serpentinites belong to the ultramafic cumulate ophiolite rocks, originally of harzburgite composition (metamorphic peridotite) that represents a fragment of the oceanic lithosphere formed in a back arc basin. The microprobe analyses of chromite show that it belongs to the aluminum chromite type. The podiform chromite is characterized by low TiO2 (<0.3 content) and MnO (<0.27 content) which are considered as distinct features for Alpine type chromite deposits (Dichey, 1975). The chromite is low in Al content in relative to Cr+3 content similar to ophiolitic pediform chromites (Lebance et al., 1980). Podiform chromites in serpentinites are related typically to SSZ ophiolites. Finally, the field and geochemical studies show that the serpentinites belong to the low temperature Alpine type ophiolites. They belong to the ophiolitic mantle sequence formed in SSZ ophiolites which were thrust over the continental margins during the collisional stage of back arc environment. SSZ ophiolites may also form in the initial stages of arc splitting and are also characterized by strongly depleted, harzburgite mantle sequences containing podiform chromite deposits and by crystallization sequences in which olivine is followed by pyroxene (Pearce et al. (1984). Acknowledgements The authors wish to express their gratitude and sincere thanks to Prof. Dr. S. El Gaby for his valuable advices and for critically reading the manuscript. References
Figure 19 Studied chromite plotted on the Cr Fe+3 + 2Ti diagram (after Jan and Windley, 1990).
+3
–Al
+3
–
tween Lat. 25°50 and 25°320 N and Long. 33°520 and 34°020 and extend NE SW trend. They are tectonically emplaced within a distinct lithologic association of schist, basic metavolcanic rocks and ophiolitic metagabbro. Talc deposits occur as lenses and shear pods of variable lengths and widths in the serperntinites and related rocks (talc carbonate). They are formed at the expense of serpentinites along shear zones during a stage of CO2-metasomatism. Most of the magnesite veins were formed during the formation of talc carbonate. Petrographically, the serpentinites are composed essentially of antigorite, chrysotile and lizardite with subordinate amounts of magnesite, talc, dolomite, chromite, and chlorite. They show mesh, bastite and knitte textures. The presence of clinochrysotile as cross- feber veinlets traversing the other serpentinite minerals indicates a late crystallization under static conditions as a result of the activity of meteoric waters. The transformation from lizardite and chrysotile to antigorite in serpentenites took place at the beginning of low grade metamorphism (green-schist facies). The examined serpentinites have been derived from harzburgite and dunite due to the prevalence of mesh and bastite texture. Magnesite deposits occur at high levels of the serpentinites outcrop .The chromite lenses present in Wadi Um Salatit serpentinites possess a podiform shape.
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